1. Field of the Invention
The invention relates to an awake state estimation device and, more specifically, to an awake state estimation device that estimates an awake state of a driver of a mobile unit such as a motor vehicle or the like.
2. Description of the Related Art
In order to prevent a driver from falling asleep while driving, various awake state estimation devices for estimating an awake state of the driver have been devised. An awake state estimation device described in Japanese Patent Application Publication No. 9-277848 (JP-A-9-277848) estimates the awake state of a driver on the basis of a steering frequency obtained by analyzing a steering angle signal that shows the steering angle of the vehicle. Then, the awake state estimation device performs different estimation processes according to the type of the path on which the vehicle is traveling.
More concretely, when it has been determined that the path on which the vehicle is traveling is a straight path, the awake state estimation device determines the awake state of the driver on the basis of a peak value of the spectrum of the steering frequency and the magnitude of the peak frequency at which the peak value occurs. A reason for this is that when a vehicle is traveling on a straight path, the peak value of the steering spectrum and the frequency (peak frequency) at which the peak value occurs change greatly according to the awake state.
On the other hand, when it has been determined that the path on which the vehicle is traveling is a curved path, the awake state estimation device determines the awake state of the driver on the basis of an integrated value of the spectrum of a predetermined frequency band in the steering spectrum. A reason for this will be explained below. When a vehicle is traveling on a curved path, the peak frequency of the steering spectrum changes under the effect of the curvature of the curved path. Therefore, if the awake state is determined on the basis of the peak frequency as in the case where the vehicle is traveling on a straight path, an accurate awake state cannot be estimated. However, during the travel on a curved path, the integrated value of the spectrum of a predetermined frequency band greatly changes affected by the awake state not by the curvature of the curved path. Therefore, the awake state can be accurately estimated by estimating the awake state on the basis of the integrated value of the spectrum of a predetermined frequency band.
Then, after estimating the awake state of the driver, the awake state estimation device finds a moving average of numerical values that show the estimated awake state in a predetermined period of time, in order to absorb an instantaneous change of the awake state.
A technology different from the awake state estimation device described in Japanese Patent Application Publication No. 9-277848 (JP-A-9-277848) is a driving state detection device described in Japanese Patent Application Publication No. 11-342764 (JP-A-11-342764). The technology described in the Japanese Patent Application Publication No. 11-342764 (JP-A-11-342764) will be described below. Generally, a driver steers so that the vehicle travels on an ideal track. However, although the driver can steer the vehicle so that the vehicle travels on a substantially ideal road, it is impossible to steer so that the vehicle always travels on an ideal track. Therefore, the driver constantly corrects the traveling direction of the vehicle so that the vehicle travels on an ideal track, while driving the vehicle. The steering that the driver performs in order to constantly correct the traveling direction of the vehicle so that the vehicle travels on an ideal track is termed corrective steering. The technology described in Japanese Patent Application Publication No. 11-342764 (JP-A-11-342764) estimates a time-dependent change of the lateral acceleration that results from the ideal traveling track. Then, by eliminating the time-dependent change in the lateral acceleration estimated from an actual time-dependent change in the lateral acceleration that is measured by using the lateral acceleration sensor, a time-dependent change in the lateral acceleration that results only from the foregoing corrective steering is extracted. Then, the extracted time-dependent change in the lateral acceleration resulting only from the corrective steering is compared with a reference value determined beforehand, so as to detect a drowsy state of the driver. Besides, in the technology described in Japanese Patent Application Publication No. 11-342764 (JP-A-11-342764), the drowsy state of the driver may also be detected on the basis of the steering angle instead of the lateral acceleration.
However, the foregoing related technology has the following problems. Generally, the tendency in the foregoing corrective steering varies depending on drivers. More concretely, in the case where a vehicle is traveling on a curved path, the fluctuation of the corrective steering is small if the driver has good driving skill, and is larger if the driver has poor driving skill. If drivers vary in the driving skill, the result of estimation of the awake state is further affected by the traveling on a curved path. It is only natural that drivers vary in driving skill. That is, of the foregoing related technologies, the technology described in Japanese Patent Application Publication No. 9-277848 (JP-A-9-277848) is not capable of sufficiently restraining the effect that the travel of a vehicle on a curved path has on the result of estimation of the awake state. Besides, even in the case where a certain driver drives a certain vehicle, the tendency of the fluctuation of the corrective steering generally differs according to the kinds of curved paths on which the vehicle travels. Therefore, the effect that travel of the vehicle on a curved path has on the result of estimation of the awake state cannot be sufficiently restrained even by the technology described in Japanese Patent Application Publication No. 11-342764 (JP-A-11-342764) that detects the drowsy state of the driver by using the lateral acceleration which results only from the corrective steering, or the steering angle that results only from the corrective steering, that is, the foregoing corrective steering angle.